Advances in information technology result in rapidly increasing demands for display devices having various shapes. Correspondingly, various flat panel display devices, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), electro luminescent display (ELD) devices, vacuum fluorescent display (VFD) devices, and the like, have been continuously investigated, and some of them have been already applied to various apparatuses in practice.
Among these flat panel display devices, the LCD devices are most widely used for a mobile image display device in place of CRT monitors in view of their merits including excellent image quality, light weight, compactness, and low power consumption. Specifically, the LCD devices are developed for monitors of TV sets which can receive and display broadcasting signals, and monitors of computers in addition to mobile display devices such as notebook computers.
In order to allow the LCD devices to be applied to various apparatuses as a general screen display device, it is necessary for the LCD devices to realize high quality images such as high definition, high brightness, large size while maintaining merits such as light weight, compactness, and low power consumption.
Generally, the LCD device comprises a first substrate, a second substrate, a liquid crystal layer interposed between the first and second substrates which are assembled to each other with a predetermined space defined therebetween.
More specifically, the first substrate is formed with a plurality of gate lines arranged at uniform intervals in one direction, and a plurality of data lines arranged at uniform intervals in another direction perpendicular to the gate lines to define pixel regions. Each of the pixel regions has a pixel electrode formed therein, and a thin film transistor formed at a region where the respective gate lines and the data lines cross each other, and acting to transmit a data signal of the data lines to an associated pixel electrode according to a signal applied to the gate lines.
In addition, the second substrate is formed with a black matrix layer to shield light from a portion excluding the pixel regions, a color filter layer of R, G and B to exhibit color images, and a common electrode on the color filter layer to realize the images.
For such an LCD device, liquid crystals are arranged in the liquid crystal layer interposed between the first and second substrates by electric field generated between the pixel electrode and the common electrode such that an image is exhibited by adjusting an amount of light transmitting through the liquid crystal layer according to an orientation degree of the liquid crystal layer.
Such an LCD device is referred to as a twisted nematic (TN) mode LCD device. Since the TN mode LCD device has a disadvantage in terms of a narrow view angle, an in-plane switching (IPS) mode LCD device was developed to overcome the disadvantage of the TN mode LCD device.
For the IPS mode LCD device, the pixel electrode and the common electrode are formed, and parallelly separated a predetermined distance from each other on the pixel region of the first substrate to generate a horizontal electric field between the pixel electrode and the common electrode such that the liquid crystal layer is oriented by the electric field.
Meanwhile, the LCD device further comprises a spacer formed between the first and second substrate to maintain a constant separation therebetween where the liquid crystal layer is formed.
The spacer can be, classified into a ball spacer, and a column spacer according to the shapes thereof.
The ball spacers have a spherical shape, and are scattered on the first and second substrates. The ball spacers move relatively freely even after the first and second substrates are assembled, and have a small contact area with the first and second substrates.
On the other hand, the column spacer is formed by an array process on the first substrate or the second substrate, and is secured to one of the substrates in a column shape having a predetermined height. Accordingly, the column spacer has a higher contact area than that of the ball spacers.
A liquid crystal display device having a conventional column spacer will be described with reference to the drawings.
FIG. 1 is a cross-sectional view illustrating the liquid crystal display device having the column spacer.
Referring to FIG. 1, the LCD device having the column spacer comprises first and second substrates 30 and 40 facing each other, a column spacer 20 formed between the first and second substrates 30 and 40, and a liquid crystal layer (not shown) filled between the first and second substrates 30 and 40.
The first substrate 30 is formed thereon with gate lines 31 and data lines (not shown) vertically crossing each other to define pixel regions, each of which has a pixel electrode (not shown) formed therein, and thin film transistors (TFT) at respective regions where the gate lines 31 cross the data lines.
The second substrate 40 is formed thereon with a black matrix layer 41 at a region excluding the pixel regions, a color filter layer 42 of a stripe shape corresponding to the pixel regions on a longitudinal line horizontal to the data lines, and a common electrode or an overcoat layer 43 on the overall surface.
Here, the column spacer 20 is formed at a predetermined position above the gate lines 31.
In addition, the first substrate 30 is further formed with a gate insulation layer 36 over an entire upper surface including the gate lines 31, and a passivation layer 37 on the gate insulation layer 36.
FIGS. 2A and 2B are a plan view, and a cross-sectional view illustrating a touch defect on the liquid crystal display device comprising the column spacer.
As shown in FIGS. 2A and 2B, for the liquid crystal display device comprising the conventional column spacer described above, if an operator touches a surface of a liquid crystal panel 10 in a predetermined direction with a finger or other articles, a spot is formed on a touched portion. Since it is generated by touching the surface of the panel, the spot is called a touch spot, or since the spot is observed on a screen, it is also called a touch defect.
Such a touch defect is considered to be created due to a large frictional force resulting from a greater contact area between the column spacer 20 and the first substrate 1 facing each other in comparison with the structure employing the ball spacers. That is, since the column spacer 20 has a columnar shape, and has a great contact area with respect to the first substrate 1 compared with the ball spacers, as shown in FIG. 2B, it takes a longer period of time for the touched portion to recover an original state after shift of the first substrate 1 with respect to the second substrate 2 caused by touch on the surface, thereby allowing the spot to remain on the touched portion until the touched portion recovers the original state.
However, such a liquid crystal comprising the conventional column spacer has problems as follows.
First, since the contact area between the column spacer and the substrate facing each other is wide, causing a high frictional force, it takes a long period of time for the touched portion to recover the original state when one of the substrates are shifted with respect to the other due to the touch, thereby causing the spot to be observed while the touched portion is recovered to the original state.
Second, when the liquid crystal panel comprising the column spacer is brought into a high temperature condition in an upright state, liquid crystals suffer from thermal expansion. In severe cases, the cell gap expands thicker than a height of the column spacer, and causes the liquid crystals to flow down to a lower portion of the liquid crystal panel so that the lower portion of the liquid crystal panel bulges, and looks opaque.